The invention relates to a low-alloyed structural steel for desalination plants having improved corrosion behaviour, especially with respect to sea water and a method of making it.
Unalloyed steels corrode to different degrees when subjected to sea water, brackish water, polluted fresh water, the atmosphere and the soils in the area of such waters, depending on the concentration, the pH-value, the gas content (in particular the oxygen content), the flow rate and the temperature of the aggressive media. For this reason construction parts and structures of unalloyed steels, such as ships and other marine structures, are protected against corrosion by a coating.
Often, however, technical and economical considerations do not permit corrosion protection by coating, or they require a corrosion-inhibiting behaviour from the base material in event of damage to the coating.
The influence of corrosion can impair the safety and functioning of a structure (e.g. by reducing the supporting cross section, by cracks, leaks and the like). In certain cases of application the amount of the corrosion products formed can also negatively affect the functioning of a structure, e.g. the clogging of heat exchanger tubes and pipes in sea water desalination plants or cooling systems.
Therefore, a material in contact with corroding media ought to have an improved behaviour against plane corrosion as well as against local corrosion.
As is known, the inherent corrosion protection of steels is caused by the formation of a more or less dense protective layer on the surface. In high alloyed corrosion and acid resistant steels, this protective layer consists of a thin, but very dense, adhering layer of metal oxide that is largely resistant to chemical media, the so-called "passive" layer. Low alloyed steels can also be protected against atmospheric corrosion by forming covering layers, which, however, cannot be compared with a passive layer. These comparatively thick, but increasingly dense covering layers are formed by initial corrosion in the course of numerous wet and dry periods and consist mainly of water-insoluble crystallized corrosion products. Although these layers are not as dense as closed oxide layers, they still prevent or reduce, the admission of oxygen to the surface of the steel. With these low alloyed steels which are known as weather-proof structural steels the possibility of forming a protective layer is largely lost when the steel is constantly immersed in water or the protective effect of the covering layer is largely lost when the aggressive media contains chlorine.
For the last mentioned corrosion conditions, high alloyed steels that form a passive layer, have been the only materials hitherto available. But for economical reasons it is not always possible to use them. On the other hand, the contents of the passivating alloying elements required is so high that the production and processing of such steels is limited.